The present invention relates to two communication methods in communication systems, a computation unit that is set up to be incorporated into a communication system, a communication system, a computer program and a machine-readable storage medium.
Known packet-based communication systems comprise a plurality of nodes, a central entity (also called the “master” or “coordinator”) and a jointly used transmission medium (also referred to as a “shared transmission medium” or “shared medium”).
A basic problem for a packet-based communication system of this kind is controlling access to the “shared medium”, because when the same resources are simultaneously being used by various nodes it is normally very unlikely the transmission will be successful (i.e. error-free). In this context, there are a large number of different degrees of freedom and possible manifestations and also many different optimization directions, for example for throughput, fairness, real-time capability, etc.
In general, it is possible to distinguish between competing and coordinated media access methods.
Competing methods can fundamentally involve two or more nodes simultaneously accessing the “shared medium” and wishing to transmit corresponding data, which then results in a collision on the shared transmission medium. Examples of this kind of method are (slotted) ALOHA, carrier sense multiple access (CSMA) and CSMA with collision detection (CSMA/CD, used in the Ethernet variants 10BASE5 and 10BASE2, inter alia).
In the case of competing media access methods such as CSMA/CD, very fast access to the “shared medium” is basically possible, but at the same time, even in the error-free case, there is no guarantee of successful access (in the sense of access that entails a successful transmission) being able to be effected within a prescribed time. The reason for this is that even during regular operation it is possible for collisions to arise at any time, which normally brings about an erroneous transmission. This probability of collision normally rises as the number of nodes increases and the volume of data increases, i.e. it correlates to the utilization level of the shared transmission medium. Therefore, such methods are generally unsuitable or have only limited suitability for real-time-critical data, which need to be transmitted within a certain time.
Furthermore, competing methods of this kind can sometimes alternatively be regarded as very critical if all or at least some of the nodes need to be supplied with power via the “shared medium” (e.g. via a wired bus), or if all or at least some of the nodes are powered by the same energy source (e.g. a battery). The reason is that in such a case, depending on the system design, it will be possible for the whole system or a portion thereof to fail on account of an excessively high power requirement if too many nodes wish to access the “shared medium” simultaneously. The reason for this is essentially that the power draw of a node is normally higher in transmission mode than in reception mode or in non-transmission mode.
Unlike in the case of competing methods, coordinated media access methods should not, in the error-free case, encounter any collisions as a result of a plurality of nodes simultaneously accessing the “shared medium”. In this case, the media access can be coordinated statically, as in the case of static time-division multiplexing methods, for example, in which each node periodically has exclusive use of the “shared medium” for transmission for a certain time. By way of example, this can be effected using a statically stipulated schedule, which is known to all nodes, in conjunction with time synchronization of all nodes. Alternatively, the coordination can be implemented with the aid of control by a central entity, as in the case of a classic polling method, for example. Furthermore, distributed coordination is alternatively possible, this being used for methods such as “token ring” or “token bus”, for example.
Coordinated methods normally have a more deterministic (i.e. more easily predictable) behavior than competing methods, and it is usually possible, at least to a certain extent, to guarantee maximum access times for the “shared medium”. Particularly in the case of methods in which the coordination is effected by a central entity, these guaranteed maximum access times may be relatively long, however, particularly when there are a large number of nodes and/or there is a high volume of traffic. In the case of a cyclic polling method, for example, the central entity cyclically awards each node access to the “shared medium”, with the maximum cycle time and hence the guaranteed maximum access time for the “shared medium” normally being directly proportional to the number of nodes and to the maximum possible volume of traffic.
Conventional coordinated methods can therefore be used to make heavy timing demands on at least some of the data to be transmitted (e.g. in the sense of comparatively short maximum access times) only for systems with a relatively small number of nodes.
A further subject for the design and realization of communication systems with a central control/supervisory entity (e.g. in the case of corresponding systems with coordinated media access methods) is the supply of said central entity with information about the nodes, for example in respect of the current state of a node or in respect of particular parameters. In conventional systems, this usually involves the interchange of special control messages between the central entity and the individual nodes. These control messages are in this case normally made up of a suitable preamble for synchronization, a message header and the actual control data and therefore often have a very large overhead, particularly if only one or a few information/control bit(s) need(s) to be transmitted. Furthermore, a node first of all needs to be provided with access to the “shared medium” before such a message is transmitted. The options already presented above come into consideration for this, in principle, but the outlay for obtaining even simple information contents is therefore often relatively high.
It is an object of the present invention to provide a communication method for a communication system that allows a high level of transmission reliability for relatively low outlay and, in particular, is suited to being used in communication systems having a large number of nodes. It is additionally an object to allow fast querying of states (particularly transmission requirements) for simultaneous minimum power draw by the nodes.